This invention relates to interface circuitry for interconnecting a two-wire communication channel with a four-wire communication channel. More particularly, the invention is directed to a telephone subscriber line interface circuit (SLIC) of hybrid form which provides both direct-current power and telephonic signals to a subscriber line while rejecting noise from a source of the power.
In communication systems, particularly telephony, it is common practice to transmit signals between a subscriber station and a central switching office via a two-wire bidirectional communication channel. Between central offices, communication is implemented frequently by use of a four-wire communication channel comprising a pair of two-wire channels for communicating in opposite directions. An interface circuit, or SLIC, connects the two-wire channel with the four-wire channel. The SLIC directs the signal transmission between the two-wire bidirectional channel and the input and the output portions of the four-wire communication channel while inhibiting any coupling of incoming and outgoing communications between the corresponding portions of the four-wire communication channel. The function of inhibiting the coupling of the incoming and outgoing portions of the four-wire communication channel is known as a transhybrid balance function.
One form of interface circuit is taught in U.S. Pat. No. 4,041,252 issued in the name of Earl T. Cowden and assigned to the assignee of the present invention. The Cowden circuit and variations thereof have been widely and successfully utilized in the system 1210 electronic switching system manufactured and sold by International Telephone and Telegraph Corporation. The Cowden circuit includes provisions for the injection of direct current into the subscriber line from a battery or other source of power, and also provides for the injection of voice signals into the two-wire subscriber line as well as reception of voice signals therefrom.
A further interface circuit is disclosed in a co-pending U.S. patent application entitled ELECTRONIC HYBRID HAVING SYNTHESIZED IMPEDANCE CIRCUITRY, filed in the name of Philip T. Martin on Dec. 22, 1983 and having Ser. No. 564,637, now U.S. Pat. No. 4,567,331. Both the Cowden and Martin circuits employ feedback circuitry and resistive feed of direct current into each wire of the subscriber channel. The feedback circuitry of the Martin circuit is more extensive than that of the Cowden circuit and provides for a synthesized impedance which is presented to the subscriber channel. With respect to the resistive connections, the subscriber line may be characterized by a termination impedance of 900 ohms AC (alternating current) in which case each of the feed resistors would have a value of 450 ohms for a matching of the impedance.
In the foregoing Martin circuit, the use of feedback to synthesize the desired impedance presented to the subscriber line results in a lowered value of resistance to the feed resistors and, hence, a reduction in power dissipated in the interface circuit.
In view of the connection of the interface circuit between a source of power and the subscriber line, there is the danger than any noise which may be present on the power lines may be coupled through the interface circuit to the subscriber line. The presence of such noise on the subscriber line is undesirable, particularly in that it may degrade the quality of voice signals and other communication carried by the subscriber line.